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iterator: optimize merged iterator (again) #421

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iterator: optimize merged iterator (again) #421

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162 changes: 101 additions & 61 deletions leveldb/iterator/merged_iter.go
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Original file line number Diff line number Diff line change
Expand Up @@ -7,8 +7,6 @@
package iterator

import (
"container/heap"

"github.com/syndtr/goleveldb/leveldb/comparer"
"github.com/syndtr/goleveldb/leveldb/errors"
"github.com/syndtr/goleveldb/leveldb/util"
Expand Down Expand Up @@ -36,8 +34,7 @@ type mergedIterator struct {
errf func(err error)
releaser util.Releaser

indexes []int // the heap of iterator indexes
reverse bool //nolint: structcheck // if true, indexes is a max-heap
heap indexHeap
}

func assertKey(key []byte) []byte {
Expand Down Expand Up @@ -72,20 +69,19 @@ func (i *mergedIterator) First() bool {
return false
}

h := i.indexHeap()
h.Reset(false)
i.heap.Reset()
for x, iter := range i.iters {
switch {
case iter.First():
i.keys[x] = assertKey(iter.Key())
h.Push(x)
i.heap.Add(x)
case i.iterErr(iter):
return false
default:
i.keys[x] = nil
}
}
heap.Init(h)
i.heap.Init(false)
i.dir = dirSOI
return i.next()
}
Expand All @@ -98,20 +94,19 @@ func (i *mergedIterator) Last() bool {
return false
}

h := i.indexHeap()
h.Reset(true)
i.heap.Reset()
for x, iter := range i.iters {
switch {
case iter.Last():
i.keys[x] = assertKey(iter.Key())
h.Push(x)
i.heap.Add(x)
case i.iterErr(iter):
return false
default:
i.keys[x] = nil
}
}
heap.Init(h)
i.heap.Init(true)
i.dir = dirEOI
return i.prev()
}
Expand All @@ -124,31 +119,29 @@ func (i *mergedIterator) Seek(key []byte) bool {
return false
}

h := i.indexHeap()
h.Reset(false)
i.heap.Reset()
for x, iter := range i.iters {
switch {
case iter.Seek(key):
i.keys[x] = assertKey(iter.Key())
h.Push(x)
i.heap.Add(x)
case i.iterErr(iter):
return false
default:
i.keys[x] = nil
}
}
heap.Init(h)
i.heap.Init(false)
i.dir = dirSOI
return i.next()
}

func (i *mergedIterator) next() bool {
h := i.indexHeap()
if h.Len() == 0 {
if i.heap.Empty() {
i.dir = dirEOI
return false
}
i.index = heap.Pop(h).(int)
i.index = i.heap.Top()
i.dir = dirForward
return true
}
Expand Down Expand Up @@ -177,22 +170,22 @@ func (i *mergedIterator) Next() bool {
switch {
case iter.Next():
i.keys[x] = assertKey(iter.Key())
heap.Push(i.indexHeap(), x)
i.heap.FixTopWith(x)
case i.iterErr(iter):
return false
default:
i.keys[x] = nil
i.heap.Pop()
}
return i.next()
}

func (i *mergedIterator) prev() bool {
h := i.indexHeap()
if h.Len() == 0 {
if i.heap.Empty() {
i.dir = dirSOI
return false
}
i.index = heap.Pop(h).(int)
i.index = i.heap.Top()
i.dir = dirBackward
return true
}
Expand All @@ -209,37 +202,37 @@ func (i *mergedIterator) Prev() bool {
case dirEOI:
return i.Last()
case dirForward:
key := append([]byte(nil), i.keys[i.index]...)
h := i.indexHeap()
h.Reset(true)
i.heap.Reset()
for x, iter := range i.iters {
if x == i.index {
i.heap.Add(x)
continue
}
seek := iter.Seek(key)
seek := iter.Seek(i.keys[i.index])
switch {
case seek && iter.Prev(), !seek && iter.Last():
i.keys[x] = assertKey(iter.Key())
h.Push(x)
i.heap.Add(x)
case i.iterErr(iter):
return false
default:
i.keys[x] = nil
}
}
heap.Init(h)
i.heap.Init(true)
}

x := i.index
iter := i.iters[x]
switch {
case iter.Prev():
i.keys[x] = assertKey(iter.Key())
heap.Push(i.indexHeap(), x)
i.heap.FixTopWith(x)
case i.iterErr(iter):
return false
default:
i.keys[x] = nil
i.heap.Pop()
}
return i.prev()
}
Expand All @@ -266,7 +259,8 @@ func (i *mergedIterator) Release() {
}
i.iters = nil
i.keys = nil
i.indexes = nil
i.heap.indexes = nil
i.heap.keys = nil
if i.releaser != nil {
i.releaser.Release()
i.releaser = nil
Expand All @@ -292,10 +286,6 @@ func (i *mergedIterator) SetErrorCallback(f func(err error)) {
i.errf = f
}

func (i *mergedIterator) indexHeap() *indexHeap {
return (*indexHeap)(i)
}

// NewMergedIterator returns an iterator that merges its input. Walking the
// resultant iterator will return all key/value pairs of all input iterators
// in strictly increasing key order, as defined by cmp.
Expand All @@ -307,44 +297,94 @@ func (i *mergedIterator) indexHeap() *indexHeap {
// won't be ignored and will halt 'merged iterator', otherwise the iterator will
// continue to the next 'input iterator'.
func NewMergedIterator(iters []Iterator, cmp comparer.Comparer, strict bool) Iterator {
keys := make([][]byte, len(iters))
return &mergedIterator{
iters: iters,
cmp: cmp,
strict: strict,
keys: make([][]byte, len(iters)),
indexes: make([]int, 0, len(iters)),
iters: iters,
cmp: cmp,
strict: strict,
keys: keys,
heap: indexHeap{
indexes: make([]int, 0, len(iters)),
keys: keys,
cmp: cmp,
},
}
}

// indexHeap implements heap.Interface.
type indexHeap mergedIterator
// indexHeap provides heap operations for indexes.
// It specializes 'heap' with int element type.
type indexHeap struct {
indexes []int
keys [][]byte
cmp comparer.Comparer
reverse bool
}

func (h *indexHeap) Len() int { return len(h.indexes) }
func (h *indexHeap) Less(i, j int) bool {
i, j = h.indexes[i], h.indexes[j]
r := h.cmp.Compare(h.keys[i], h.keys[j])
if h.reverse {
return r > 0
func (h *indexHeap) Init(reverse bool) {
h.reverse = reverse
// heapify
n := len(h.indexes)
for i := n/2 - 1; i >= 0; i-- {
h.down(i, n)
}
return r < 0
}

func (h *indexHeap) Swap(i, j int) {
h.indexes[i], h.indexes[j] = h.indexes[j], h.indexes[i]
func (h *indexHeap) Reset() {
h.indexes = h.indexes[:0]
}

func (h *indexHeap) Push(value interface{}) {
h.indexes = append(h.indexes, value.(int))
func (h *indexHeap) Add(x int) {
h.indexes = append(h.indexes, x)
}

func (h *indexHeap) Pop() interface{} {
e := len(h.indexes) - 1
popped := h.indexes[e]
h.indexes = h.indexes[:e]
return popped
func (h *indexHeap) Empty() bool { return len(h.indexes) == 0 }
func (h *indexHeap) Top() int { return h.indexes[0] }

func (h *indexHeap) Pop() int {
top := h.indexes[0]
n := len(h.indexes) - 1
h.swap(0, n)
h.down(0, n)
h.indexes = h.indexes[:n]
return top
}

func (h *indexHeap) Reset(reverse bool) {
h.reverse = reverse
h.indexes = h.indexes[:0]
func (h *indexHeap) FixTopWith(x int) {
// replace top
h.indexes[0] = x
// and then fix
h.down(0, len(h.indexes))
}

func (h *indexHeap) down(i0, n int) bool {
i := i0
for {
j1 := 2*i + 1
if j1 >= n || j1 < 0 { // j1 < 0 after int overflow
break
}
j := j1 // left child
if j2 := j1 + 1; j2 < n && h.less(j2, j1) {
j = j2 // = 2*i + 2 // right child
}
if !h.less(j, i) {
break
}
h.swap(i, j)
i = j
}
return i > i0
}

func (h *indexHeap) less(i, j int) bool {
i, j = h.indexes[i], h.indexes[j]
r := h.cmp.Compare(h.keys[i], h.keys[j])
if h.reverse {
return r > 0
}
return r < 0
}

func (h *indexHeap) swap(i, j int) {
h.indexes[i], h.indexes[j] = h.indexes[j], h.indexes[i]
}
13 changes: 11 additions & 2 deletions leveldb/iterator/merged_iter_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -61,8 +61,7 @@ var _ = testutil.Defer(func() {
})
})

func BenchmarkMergedIterator(b *testing.B) {
n := 11
func benchmarkMergedIteratorN(b *testing.B, n int) {
iters := make([]Iterator, n)
for i := range iters {
kv := testutil.KeyValue_Generate(nil, 100, 1, 1, 10, 4, 4)
Expand All @@ -79,3 +78,13 @@ func BenchmarkMergedIterator(b *testing.B) {
}
}
}

func BenchmarkMergedIterator(b *testing.B) {
b.Run("2 iters", func(b *testing.B) {
benchmarkMergedIteratorN(b, 2)
})

b.Run("50 iters", func(b *testing.B) {
benchmarkMergedIteratorN(b, 50)
})
}